It may be particularly desirable to reduce the size of a mobile wireless communications device, for example, to a thinner form factor. Reducing the size of a device generally requires thinner substrates and components coupled to the substrates. Additionally, it may also be desirable to reduce fabrication costs, for example, by providing a simpler fabrication process. For example, one approach to reducing substrate fabrication costs may include the use of a double-sided flexible substrate and a solder mask or coverlay, as a less expensive option compared to a multilayer flexible substrate and a coverlay.
Additionally, increased use of relatively high frequency radar devices in the automotive industry (e.g., parking sensors, backup sensors, blind spot sensors, etc.) coupled with the desire for non-hermetic packaging driven by high volume cost concerns may also increase the desire for lightweight, low cost, high frequency capable packaging and substrate materials. Liquid crystal polymer (LCP) may be considered one such material, for example.
The use of thinner components may generally require the use of a higher percentage of bare die and/or flip chip components, for example. Accordingly, mounting those components to a substrate without increasing fabrication costs may be increasingly a concern. Ultrathin flexible circuits with reduced thickness active and passive components may thus be used to address this concern. Thin film passive devices, for example thin film resistors, have generally been available for several decades. However, a thin film passive device typically must be embedded within a multilayer printed circuit board (PCB), which may not be possible, for example, with certain flexible circuits.
Existing coverlay materials for flexible circuits generally require an adhesive-based bonding that includes relatively large apertures to be opened around pads for subsequent electrical interconnection between the flexible circuit and the electronic assembly. These coverlay materials hence generally allow for the addition of little if any additional functionality outside of protection of the underlying features on the flex circuit. Accordingly, additional fabrication methods to include additional device functionality are needed.